1. Field of the Invention
The present invention relates generally to a method of performing a handover between heterogeneous mobile communication systems, and in particular, to a method of measuring received signal strength and maintaining synchronization for a handover between heterogeneous mobile communication systems.
2. Description of the Related Art
In general, a 3rd generation (3G) mobile communication system refers to a CDMA (Code Division Multiple Access) mobile communication system that supports packet service and voice service. CDMA systems are categorized into asynchronous CDMA, i.e. 3GPP (3rd Generation Project Partnership) or UMTS (Universal Mobile Telecommunication System), adopted in Europe, and synchronous CDMA, i.e. 3GPP2 or CDMA2000, deployed in the United States. The 3GPP has proposed FDD (Frequency Division Multiplexing) that distinguishes downlink (DL) transmission/reception from uplink (UL) transmission/reception by frequency, and TDD (Time Division Duplexing) that distinguishes DL transmission/reception from UL transmission/reception by time. The TDD is further branched into WB-TDD (Wide Band-TDD) using 3.84 Mcps and NB-TDD (Narrow Band-TDD) using 1.28 Mcps.
2nd generation (2G) mobile communication systems include GSM (Global System for Mobile communications) and IS (Interim Standard)-95. The GSM, which was deployed throughout Europe in 1992, provides service in TDMA (Time Division Multiple Access). Meanwhile, the IS-95, which was deployed in Korea and the U.S., provides service in CDMA.
As described above, the 2G and 3G mobile communication systems employ different frequencies and communication schemes. Such heterogeneous systems may be in service in adjacent areas. Specifically, as the 3G mobile communication systems have been in service, the service areas of FDD, WB-TDD, NB-TDD, GSM, CDMA2000, and IS-95 occasionally lie adjacent to each other or overlap with each other.
One significant feature of the 3G mobile communication systems is supporting global roaming irrespective of communication scheme or frequency. This implies that a handover to a different 2G system or to a different 3G generation system is supported. When a UE (User Equipment) moves from one service area to another using a different communication scheme or frequency, a handover is essential for global roaming.
As the above various communication services have been in service, there is a need for a method of performing a handover between heterogeneous mobile communication systems using different communication schemes and frequencies.
Handover between service areas using different communication schemes is defined as “inter-RAT (inter-Radio Access Technologies) handover,” and monitoring the states of Node Bs for the inter-RAT handover is termed “inter-RAT measurement.” Different frequencies can be used between neighbor Node Bs using the same communication scheme. Handover between these Node Bs is defined as “inter-frequency handover” and monitoring the states of the Node Bs for the inter-frequency handover is called “inter-frequency measurement”.
Inter-frequency or inter-RAT measurements are performed in various ways in the 3G mobile communication systems. One way is that a UE discontinues communication with a serving Node B for a predetermined time and measures a downlink channel received in a different frequency band or belonging to a different radio access technology. Another way is that the UE transmits data at a higher rate and measures the downlink channel for the spared time. A third way is that the UE measures the downlink channel, continuing the ongoing call at a lower rate, thus with a degraded data quality.
The above-described inter-frequency and inter-RAT measurement methods are applied differently depending on whether a 3G mobile communication system identifies downlink/uplink transmission/reception by frequency or by time. Particularly, when the downlink transmission is distinguished from the uplink transmission by time, the UE carries out an inter-frequency or inter-RAT measurement for a non-downlink/uplink transmission period.
FIG. 1A illustrates a structure of a typical NB-TDD frame. Referring to FIG. 1A, an NB-TDD frame 101 is 10 ms in duration, containing 12,800 chips in view of the data rate used in NB-TDD, 1.28 Mcps. The NB-TDD frame 101 is divided into two 5-ms sub-frames of the same structure. A sub-frame 102 is comprised of 7 time slots (TSs), a DL pilot time slot (DWPTS) 104, a UL pilot time slot (UpPTS) 106, and a guard period (GP) 105. Each TS includes 864 chips. The 7 TSs are used for uplink (UL) transmission or downlink (DL) transmission. For example, ↑ denotes UL TSs and ↓ denotes DL TSs. The number of UL TSs and the number of DL TSs in a sub-frame are determined according to the ratio of DL data rate to UL data rate in the system. The first TS (TS #0) 103 is always a DL TS and the second TS (TS #1) is always a UL TS. The 96-chip DwPTS 104, the 96-chip GP 105, and the 160-chip UpPTS 106 are interposed between TS #0 and TS #1. The DwPTS 104 is used for initial cell search, synchronization to system timing, and channel estimation in a UE. The UpPTS 106 is used for channel estimation and UL synchronization in a Node B. The GP 105 eliminates interference caused by multipath delay between transmissions of the DwPTS 104 and the UpPTS 106. In an NB-TDD system, a switching point is defined for transition between the UL and DL transmissions and two switching points are set in one sub-frame. A first switching point is fixed between the DwPTS 104 and the UpPTS 106 and a second switching point is at an appropriate position between TS #1 and TS #6 according to the ratio of DL data rate to UL data rate. A primary common control physical channel (P-CCPCH) 107 is delivered using two codes in TS #0. DL channels (or UL channels) sharing the same TS are identified by their codes in the NB-TDD system. These codes are orthogonal codes of length 16.
FIG. 1B illustrates the P-CCPCH in TS#0 and the DwPTS. Referring to FIG. 1B, a P-CCPCH 107, which is a physical channel delivering a broadcasting channel (BCH) containing system information about a Node B, is comprised of a first data area 109, a midamble 110, a second data area 111, and a GP 112. The GP 112 is positioned at the end of TS #0 and includes 16 chips. The GP 112 eliminates interference from a signal in an adjacent TS. The DwPTS 104 is comprised of a GP 113 and a SYNC-DL (synchronization-downlink) code 114. The GPs 112 and 113 collectively form a 48-chip guard period to eliminate interference caused by multipath delay between TS#0 and the DwPTS 104. This guard period is relatively long because the SYNC-DL code 114 plays a very significant role. The SYNC-DL code 114 is the first signal for the UE to search for when the UE is in the service area of the NB-TDD communication system. The SYNC-DL code 114 is used for initial cell search and synchronization to a searched cell. When signals in TS #0 cause interference, the interference may have serious influence on reception of the SYNC-DL code 114. Thus, the GPs 112 and 113 are formed to have 48 chips to guarantee stable reception of the SYNC-DL code 114 in the UE. 32 SYNC-DL codes are available. The UE decides the SYNC-DL code 114 by correlating the 32 available SYNC-DL codes with a current strongest received signal and synchronizes its timing to a cell that the UE belongs to.
FIG. 2A illustrates a structure of a typical GSM frame, FIG. 2B illustrates FCCH (Frequency Correction Channel) positions and SCH (Synchronization Channel) positions in GSM frames, and FIG. 2C illustrates structures of an FCCH and an SCH illustrated in FIG. 2B. The FCCH and the SCH are used to acquire a GSM frequency and synchronize to system timing.
The GSM system is a major asynchronous 2G mobile communication system adopting TDMA. Referring to FIG. 2A, a multiframe 201 is the largest radio transport unit containing 51 frames in the GSM system. Each frame 202 has 8 TSs.
The GSM system uses the FCCH and the SCH to perform measurements for initialization and handover and to acquire synchronization between a UE and a Node B. Referring to FIG. 2B, the FCCH is delivered in the first TS 204 of each of frame #0, frame #10, frame #20, frame #30, and frame #40 among 51 frames 203. The SCH is delivered in the first TS 205 of each of frame #1, frame #11, frame #21, frame #31, and frame #41 among the 51 frames 203.
The structures of the FCCH 206 and the SCH 211 are illustrated in more detail in FIG. 2C. Referring to FIG. 2C, a SCH 211, which supplies a UE with training sequence it needs to be able to demodulate the information coming from the Node B is comprised of two TB (Tail Bit) slots 212 216, two Encrypted Bit slots 213 215, a synchronization sequence 214, and a GP(Guard Period) 217. The TB slots 212 216 consists of three bits at the beginning and the end of the SCH 211 and is used as guard time. The Encrypted Bit slots 213 215 contain the actual transmitted signaling data or user data. Also, a FCCH 206, which provides a UE with the frequency reference of the system is comprise of two TB (Tail Bit) slots 207 209, a Fixed Bit sequence 208, and GP 210. The Fixed Bit sequence 208 is transmitted in the time slot to provide the means for a mobile station to synchronize with the master frequency of the system.
FIG. 3 illustrates a situation requiring inter-RAT measurement. Referring to FIG. 3, a UE 303, which is capable of communicating in an NB-TDD system and other systems, moves to a GSM Node B 302 during exchanging voice or packets with an NB-TDD Node B 301. For a handover from the NB-TDD system to the GSM system, the UE needs to perform inter-RAT measurements. As the UE 303 moves to the GSM Node B 302, it receives an inter-RAT measurement command from the NB-TDD Node B 301 and measures the strengths of signals from the GSM Node B 302. The signals can be the FCCH, the SCH, or a transport channel illustrated in FIGS. 2A, 2B, and 2C. The NB-TDD Node B 301 issues the inter-RAT command when the signal strengths from neighbor Node Bs measured in an NB-TDD frequency band at the UE 303 are insufficient for a handover, or the strengths of signals between the UE 303 and the NB-TDD 301 become weak.
After acquiring synchronization to the GSM Node B 302 and GSM system information from signal measurements on the GSM Node B 302, the UE 303 reports the signal measurements to the NB-TDD Node B 301 and continues the ongoing communication by the handover to the GSM Node B 302 according to a command from the NB-TDD Node B 301. In this case, the UE 303 already has a call established with the NB-TDD Node B 301 and has received an inter-RAT measurement command from it. However, if a call has not been established, the UE 303 can directly perform the inter-RAT measurement using necessary information in system information included in the BCH, which the NB-TDD Node B 301 delivers on the P-CCPCH.
FIG. 4 illustrates time periods when inter-RAT measurement is available in a UE supporting NB-TDD and other mobile communication schemes in the situation illustrated in FIG. 3. Referring to FIG. 4, reference numeral 401 denotes an ith sub-frame. In the ith sub-frame 401, the UE carries out UL transmission in a TS 403 and a Node B carries out DL transmission in a TS 404. The UE can carry out an inter-RAT measurement in the other TSs, that is, TSs 405, 406, 407, and 408.
However, the inter-RAT measurement is available only in TSs except for a radio frequency transition time for which the UE transitions its transmission/reception frequency to a frequency band for transmitting/receiving an inter-RAT signal and another radio frequency transition time for which the UE returns to its original frequency band. Therefore, the measurement time is TSs 405 to 408 except for the radio frequency transition times. As the measurement time is longer, the inter-RAT measurement is facilitated and measurement reliability is increased. In FIG. 4, the measurement time is determined according to the positions of DL TSs and UL TSs. If the measurement time is long, the inter-RAT measurement is reliably done. However, if the measurement time is short, the inter-RAT measurement may not be performed reliably.
Table 1 below illustrates time required for the UE to monitor the FCCH and the SCH with respect to the radio frequency transition time when the TS#1 and TS#5 illustrated in FIG. 1A are assigned to the UE.
TABLE 1Average sync time ofAverage sync time ofmonitoringmonitoringTS1:UL, TS5:DLFCCH + SCH (ms)only FCCH (ms)0.5 ms switching time5122880.6 ms switching timemeasurement failureMeasurement failureprobability: 35.47%probability: 35.47%0.7 ms switching timemeasurement failureMeasurement failureprobability: 87.50%probability: 87.50%0.8 ms switching timeCan never be synchronized (window is shorterthan one FCCH timeslot)0.9 ms switching timeCan never be synchronized (window is shorterthan one FCCH timeslot)1.0 ms switching timeCan never be synchronized (window is shorterthan one FCCH timeslot)Referring to Table 1, the UE cannot monitor the FCCH and the SCH when the radio frequency transition time is 0.8 ms or longer.
Table 2 below illustrates time required for the UE to monitor the FCCH and the SCH with respect to the radio frequency transition time when TS#1 and TS#4 illustrated in FIG. 1A are assigned to the UE.
TABLE 2Average sync time ofAverage sync time ofmonitoringmonitoringTS1:UL. TS4:DLFCCH + SCH (ms)only FCCH (ms)0.5 ms switching time3361850.6 ms switching time4642600.7 ms switching timeMeasurement failureMeasurement failureprobability: 15.94%probability: 15.94%0.8 ms switching timemeasurement failureMeasurement failureprobability: 67.97%probability: 67.97%0.9 ms switching timeCan never be synchronized (window is shorterthan one FCCH timeslot)1.0 ms switching timeCan never be synchronized (window is shorterthan one FCCH timeslot)Referring to Table 2, the UE cannot monitor the FCCH and the SCH when the radio frequency transition time is 0.9 ms or longer.
As noted from Table 1 and Table 2, it may occur that the UE cannot monitor the FCCH and the SCH at all according to the radio frequency transition time and its assigned slots. However, a UTRAN (UMTS Radio Access Network), which has commanded an inter-RAT measurement, currently has no information about the radio frequency transition time of the UE. Therefore, if the UTRAN fails to receive an inter-RAT measurement report from the UE, it cannot identify the cause, and thus, cannot take any corresponding action.
This situation will be described in more detail herein below.
FIG. 8 is a diagram illustrating a signaling procedure when an NB-TDD Node B commands a UE to perform inter-RAT measurements on signals from GSM cells. Referring to FIG. 8, a UTRAN covering the NB-TDD Node B commands the UE to perform inter-RAT measurements on neighbor GSM cells, informing the UE of the BSICs (Base transceiver Station Identity Codes) and ARFCNs (Absolute Radio Frequency Channel Numbers) of the GSM cells in step 801. At the same time, the UTRAN tells the UE how an inter-RAT measurement reporting is to be done and whether BSIC verification is required when reporting. The inter-RAT measurement is reported each time an event is generated (event-triggered mode), or periodically.
The UE then determines whether it can perform the inter-RAT measurements. If it can, the UE performs the inter-RAT measurements and reports them to the UTRAN in step 802. However, if the UE cannot perform the inter-RAT measurements, the UE transmits to the UTRAN a MEASUREMENT CONTROL FAILURE message indicating a measurement failure cause in step 803.
Measurement failure causes are specified as unsupported measurement and configuration incomplete. The former is added to the MEASUREMENT CONTROL FAILURE message when the UTRAN commands an inter-RAT measurement that the UE does not support, while the latter is added to the MEASUREMENT CONTROL FAILURE message when the UTRAN commands the alteration of an ongoing inter-RAT measurement, but the inter-RAT measurement is not done, or a time period in which communication is discontinued for an inter-RAT measurement is occupied for another inter-RAT measurement.
The actual measurement procedure of step 802 is performed differently depending on whether BSIC verification is required, and whether the inter-RAT measurement reporting is periodic or event-triggered.
For ease of description, it is assumed that an inter-RAT measurement command requests the UE to report an inter-RAT measurement with BSIC verification in an event-triggered mode. The UE measures the strengths of signals from all neighbor GSM cells in inter-RAT measurement available TSs using information about the GSM cells (i.e., BSICs and ARFCNs) received together with the inter-RAT measurement command. There is no limit on channels that are measured. The UE simply measures the strengths of arbitrary channel signals from the GSM cells in the TSs. The UE then enumerates the signal strength measurements in a descending order and monitors an FCCH and an SCH from a GSM cell transmitting the strongest signal, thereby identifying the BSIC of the cell. If the BSIC is verified, the UE decides whether a condition given together with the inter-RAT measurement command by the NB-TDD Node B is satisfied. If it is satisfied, the UE reports the signal strength measurement with the verified BSIC to the UTRAN.
The UE then verifies the BSIC of a GSM cell that transmits a second strongest signal by monitoring an FCCH and an SCH from the GSM cell. If the BSIC is successfully verified and the condition is satisfied, the UE reports the signal strength measurement with the BSIC to the UTRAN. The signal strength measurements that satisfy the condition can be reported individually or all together.
In this manner, the UE repeats the inter-RAT measurements on GSM cells whose BSICs can be verified and repeats measurement reporting. The UE also maintains synchronization to some of the GSM cells transmitting strong signals by continuous FCCHs and SCHs monitoring in order to minimize time required for synchronization when the NB-TDD Node B commands a handover to a particular GSM cell. In this event-triggered inter-RAT measurement with BSIC verification, if GSM cells satisfy the condition but their BSICs are not verified, they are excluded from measurement reporting. In other words, inter-RAT measurements are performed on only GSM cells whose BSICs are verified. Accordingly, in the case of a shortage of time to identify BSICs from SCH signals received from all neighbor GSM cells, the UE cannot verify their BSICs and the UTRAN cannot determine whether the cause of inter-RAT measurement failure is the lack of measuring time or the absence of GSM cells satisfying the given condition. As a result, the handover is failed and the UTRAN malfunctions.
Alternatively, if the inter-RAT measurement command requires periodic reporting with BSIC verification, the signal strengths and ARFCNs of corresponding GSM cells are reported even though their BSICs are not verified. In the case of an inter-RAT measurement command not requiring BSIC verification, only the signal strengths are reported.
A distinctive shortcoming of the above-described inter-RAT measurement scenario is that if the NB-TDD Node B commands an event-triggered inter-RAT measurement with BSIC verification, the UE may not verify the BSICs of neighbor GSM cells with its assigned TSs and radio frequency transition time, though it can measure signal strengths of the GSM cells. As a result, the UE fails to report the inter-RAT measurement. However, the NB-TDD Node B cannot identify the cause of the reporting failure as being unsatisfactory received signal strengths with respect to a given condition or failed BSIC verification.